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Table 2 Average response

Table 2 Average response amplitudes and standard errors of mean (in parentheses) in the young and the aged subject group. Response amplitudes (fT/cm) Left hemisphere Right hemisphere Passive Active Passive Active Sinusoid Speech Sinusoid Speech Sinusoid Speech Sinusoid Speech Young subjects 46.6 (4.7) 73.6 (7.3) 65.3 (6.2) 86.0 (7.6) 42.6 (6.5) 77.3 (6.3) 64.1 (6.4) 90.1 (8.0) Aged subjects 42.1 (4.6) 68.8 (7.5) 49.4 (5.3) 90.6 (6.9) 56.2 (11.7) 105.3 (16.2) 69.4 (10.1) 111.5 (15.8) the passive recording condition. For speech stimuli, attentional engagement enhanced the response amplitude by approximately 17% in the right hemisphere (F(1,8) = 10.21, p < 0.05). A similar enhancement was observed in the left hemisphere but this effect did not reach statistical significance (F(1,8) = 3.83; p = n.s.). The transient brain response peaked, on the average, at the latency of 576 ms (see Table 3). In the passive recording condition, the average response latencies were 571 and 579 ms to sinusoidal and speech stimuli, respectively. In the active recording condition, the latencies were 575 and 579 ms to sinusoidal and speech stimuli, respectively. Response latency did not vary significantly as a function of stimulus type (F(1,8) = 0.45, p = n.s.), recording condition (F(1,8) = 0.34, p = n.s.) or hemisphere (F(1,8) = 1.67, p = n.s.). The x/y/z ECD locations for the responses elicited by sinusoids were, on the average, 48/10/49 mm in the left and 45/9/47 mm in the right hemisphere (F(1,8) = 0.03, p = n.s.). The ECD locations for the responses elicited by the speech sounds were 49/11/ 48 mm in the left and 49/11/45 mm in the right hemisphere (F(1,8) = 0.58, p = n.s.). No differences in the source locations of the responses elicited by the two stimulus types were observed (F(1,8) = 0.35, p = n.s.). For the young subjects, the goodness-of-fits for the left- and right-hemispheric ECDs were, on average, 91% and 83%, respectively. In the young subjects, behavioral RT followed the peak latency of the transient brain response by an average delay of 342 ms for the sinusoidal and 348 ms for the speech stimuli. No differences between the RTs for sinusoidal and speech stimuli were observed (F(1,8) = 0.13, p = n.s.) and the average of correct hits was 100% to both stimulus types. 3.2. Aged subjects In the aged subjects, in the passive recording condition, the average response amplitudes elicited by the sinusoidal stimuli were 42 and 55 fT/cm in the left and the right hemisphere, respectively (F(1,8) = 2.00, p = n.s.; see Table 2). In the active recording condition, the response amplitudes were 49 and 69 fT/cm in the left and the right hemisphere, respectively (F(1,8) = 2.95, p = n.s.). The average response amplitudes in the passive condition to speech stimuli were 69 and 105 fT/cm in the left and the right hemisphere, respectively (F(1,8) = 4.84, p = n.s.). In the active condition, the responses elicited by speech had amplitudes of 91 and 112 fT/cm in the left and the right hemisphere, respectively L.E. Matilainen et al. / Clinical Neurophysiology 121 (2010) 902–911 907 (F(1,8) = 1.45, p = n.s.). The amplitude of the response to the speech stimuli was larger than that for the response to the sinusoids in both the left (F(1,8) = 23.04, p < 0.001 and F(1,8) = 86.75, p < 0.001, in the passive and the active recording condition, respectively) and the right hemisphere (F(1,8) = 17.89, p < 0.01 and F(1,8) = 25.55, p < 0.001; see Fig. 3). Attentional engagement enhanced the response amplitudes but this effect did not reach statistical significance in all conditions. For sinusoids, attentional engagement augmented the responses by approximately 17% in the left (F(1,8) = 1.22, p = n.s.) and 26% in the right hemisphere (F(1,8) = 7.18, p < 0.05). For speech stimuli, the attention effect was approximately 32% in the left hemisphere (F(1,8) = 16.58, p < 0.01) and 6% in the right hemisphere (F(1,8) = 0.45, p = n.s.). The average latency of the transient brain response in the aged was 644 ms (see Table 3). In the passive recording condition, the latencies in the left and the right hemisphere were 644 and 642 ms for the sinusoidal and 648 and 650 ms for the speech sound, respectively. In the active recording condition, the respective latencies were 644 and 631 ms for the sinusoids and 647 and 645 ms for the speech stimuli. There were no latency differences between the hemispheres (F(1,8) = 1.89, p = n.s.), stimulus types (F(1,8) = 1.61, p = n.s.) and recording conditions (F(1,8) = 2.55, p = n.s.). The ECD location for the sinusoid-elicited brain activity was, on the average, 53/9/49 mm in the left hemisphere and 52/18/ 50 mm in the right hemisphere (F(1,8) = 1.36, p = n.s.). The source locations of the responses elicited by speech were 57/12/45 mm in the left and 52/15/47 mm in the right hemisphere (F(1,8) = 1.09, p = n.s.). No differences were observed in the source locations of cortical activity elicited by sinusoids and speech stimuli (F(1,8) = 5.30, p = n.s.). The average goodness-of-fits of the ECDs obtained for the aged subjects were 87% in both the left and the right hemisphere. RTs followed the peak latency of the transient brain response by an average delay of 317 ms for sinusoids and 314 ms for speech stimuli. No differences between the RTs for sinusoids and speech were observed (F(1,8) = 0.01, p = n.s.). Sinusoidal and speech stimuli were detected with an average accuracy of 99%. 3.3. The effects of aging Table 3 Average response latencies and standard errors of mean (in parentheses) in the young and the aged subject group. The aged subjects demonstrated considerably longer response latencies (644 ms on the average) than did the young subjects Response latencies (ms) Left hemisphere Right hemisphere Passive Active Passive Active Sinusoid Speech Sinusoid Speech Sinusoid Speech Sinusoid Speech Young subjects 567.0 (23.8) 578.8 (15.8) 572.5 (20.9) 579.2 (16.5) 575.2 (22.8) 579.8 (15.3) 576.5 (22.2) 578.5 (14.4) Aged subjects 644.4 (11.7) 648.0 (9.9) 644.2 (12.2) 647.1 (9.8) 642.2 (7.9) 650.0 (7.8) 631.1 (9.6) 644.8 (7.9)

908 L.E. Matilainen et al. / Clinical Neurophysiology 121 (2010) 902–911 (576 ms; see Fig. 4). Latency differences were observed in both hemispheres and for both stimulus types and recording conditions (F(1,16) = 5.10–16.71, p < 0.01–0.05; see Table 3). Also, the responses of the aged were larger than those of the young in the right hemisphere (see Table 2) but this difference failed to reach statistical significance in either recording condition or for either stimulus type (F(1,16) = 0.20–3.78, p = n.s.). With respect to behavioral responses, the average percentage of correctly detected sinusoids was significantly lower among the aged than the young subjects (99% vs. 100%, respectively; F(1,15) = 5.08, p < 0.05). The proportion of correct hits to speech stimuli did not vary as a function of aging (F(1,15) = 1.42, p = n.s.). RTs did not vary significantly between the subject groups either for sinusoidal (F(1,15) = 0.09, p = n.s.) or for speech sounds (F(1,15) = 0.15, p = n.s.). In the left hemisphere, the location of the ECD for the brain activity elicited by speech was, on the average, some 7–8 mm medial in the young than in the aged subjects: this was observed both in the passive (x = 48 and 55 mm for the young and aged group, respectively; F(1,16) = 5.84, p < 0.05) and in the active recording condition (x = 50 and 58 mm; F(1,16) = 8.59, p < 0.01). A similar aging-related medio-lateral shift in brain activity elicited by sinusoids was observed in the right hemisphere in the passive recording condition: the ECDs for young subjects were 9 mm medial (x = 44 and 53 mm, respectively; F(1,16) = 4.99, p < 0.05) and, in addition, 10 mm more posterior (y = 8 and 18 mm; F(1,16) = 4.75, p < 0.05) than in the aged subjects. No other differences in the source locations between the two subject groups were observed (F(1,16) = 0.00–2.74, p = n.s.). The response amplitudes were augmented similarly in both subject groups as a function of attentional engagement. There were no differences in these amplitude enhancements either for sinusoidal (F(1,16) = 0.75, p = n.s. and F(1,16) = 0.13, p = n.s. in the left and right hemisphere, respectively) or speech stimuli (F(1,16) = 1.68, p = n.s. and F(1,16) = 0.71, p = n.s.). 4. Discussion Latency (ms) 680 660 640 620 600 580 560 540 520 500 Left We studied aging-related alterations and the effects of spectral complexity on the transient brain response elicited by sounds rising slowly in intensity from an inaudible to a clearly audible level. This brain response resembles the N1 (and MMN) in terms of polarity, morphology, and generator location, and also in the sense that it reflects initial large-scale activation of auditory cortex to Sinusoid Speech Sinusoid Speech Sinusoid Speech Sinusoid Speech Active Passive Active Passive Fig. 4. Latencies of the grand-averaged left- and right-hemispheric responses to sinusoids and speech in the active and the passive recording condition. The aged subjects demonstrated, on the average, longer response latencies (644 ms; dark gray bars) than did the young subjects (576 ms; light gray bars). Line segments represent standard error of the mean (SEM). Right stimulation. However, unlike the N1 and the MMN, the transient brain response does not seem to index stimulus onset and/or change but, rather, the behavioral detection of the stimulus. In both the young and the aged subjects, the sinusoidal and speech stimuli elicited a prominent transient brain response irrespective of whether the subjects were paying attention to the stimuli. However, in the aged subjects, the brain response was delayed by some 70 ms when compared to that observed in the young subjects. In both subject groups, speech sounds elicited more prominent cortical activity than sinusoids, and attentional engagement to the stimuli enhanced brain activity. 4.1. The effects of aging Young Aged Previous results on the effects of aging on the N1(m) response have suggested differences between young and aged subjects variably in response amplitudes (Amenedo and Diaz, 1999; Harkrider et al., 2005) and latencies (Pekkonen et al., 1995; Geal-Dor et al., 2006). In the present study, the aged subjects demonstrated delayed response latencies compared to those of the young subjects (on the average, response peak amplitudes were observed at 644 vs. 576 ms post-stimulus). This observation corroborates the results of a number of previous N1(m) studies, and supports the idea of aging affecting the temporal dynamics of cortical auditory processing. This aging-related slowing down in cortical processing could be due to the neuronal loss observed in several regions of the aging brain (e.g., Jernigan et al., 2001; see also Pekkonen et al., 1995; Tremblay et al., 2002; Harkrider et al., 2005). As the behavioral reaction times of the aged subjects did not differ from those of the young, deceleration was observed only at the level of auditory processing and not in the motor responses. Thus, the delayed brain responses might be due to delays along the auditory pathway rather than to overall cognitive slowing, corroborating the conclusions by Schneider et al. (2005). Further evidence of changes taking place in the neuronal circuits of the brain can be derived from the results of ECD localization: differences between the two subject groups were observed in the left hemisphere in the case of speech sound processing, and in the right hemisphere in the case of sinusoids. These findings imply that alterations in the locations of auditory cortical processing, especially in the left hemisphere, might take place in the aging brain. The connection between age-related physiological changes and speech processing should be further studied.

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